1. Field of the Invention
The present invention relates generally to a detector for serial data transmission and more particularly to a synchronous decoder for fiber-optic signals wherein data and clock rate have been encoded together.
2. Description of the Prior Art
Present day digital data transmission systems typically recover binary data by transmitting and detecting binary encoded electrical signal waveforms. Transmission of signals over fiber-optic lines offers several advantages over copper transmission lines. Thus, fiber transmission lines possess wide bandwidths which permit multiplexing a plurality of signals with minimum losses, are compact, and immune to electromagnetic interference. The simplest method of transmission is by intensity modulation of a light source which is applied to the fiber transmission line and detected by an appropriate sensor. However, transmission over appreciable distances results in phase shift due to perturbations in the optical fiber, variations in the thickness of the jacket surrounding the fiber and the effects of mechanical stimuli which result in changes in the index of refraction. As a result, it is desirable to buffer, periodically detect, and reconstruct the transmitted signal so that it replicates the original transmission. By this means, signals can be transmitted over fiber-optic lines of significant length with minimal distortion.
The choice of the code used for transmitting the fiber-optic signals is of importance in order to obtain maximum achievable data density with minimium phase distortion. Further, in applications where remote terminates or work stations are fed from a centrally disposed computer, it is desirable to transmit a clock frequency for synchronizing the remote stations with the computer. The method of encoding known as biphase, which is similar to phase modulation, frequency shift, and manchester codes, produces a waveform from which both data and clock frequency may be recovered. Biphase zero is a self-clocking signal data transmission code characterized by always changing logic state at the end of the bit time period, but if the bit is a zero, also changing at mid-bit. Thus, a train of successive logic ones would be represented by alternating logic zero and logic one bit pulses, while a logic zero would be comprised of a comparable bit period wherein the logic state changes substantially in the middle of the bit.
An apparatus for generating biphase data signals, and asynchronously recovering the data, is shown in U.S. Pat. No. 4,584,719, Fiber Optic Workstation Datalink Interface, issued Apr. 22, 1986 to B. J. Miller, which is incorporated herein by reference.
The present invention provides an apparatus for synchronously decoding biphase data in accordance with a system clock and providing an output encoded in accordance with the well known NRZ format. Thus, it is adaptable to utilization between electrical signals generated, for example, in RS-422A format, which are converted to fiber-optic signals for transmission over fiber-optic lines, and then detected by the present invention, and the NRZ output decoded to regenerate the electrical signals.